1. Field of the Invention
The present invention relates to a starter motor for starting an internal combustion engine mounted on, for instance, an automobile. More particularly, it relates to a miniaturized and weight-saving starter motor of an overhang type wherein a pinion is fitted to a rotary output shaft by means of a linear spline structure.
2. Discussion of Background
There has been known a starter motor of an overhang type which facilitates attachment to a vehicle engine. In particular, there has been proposed a starter motor called a coaxial type starter wherein a motor section and an electromagnetic switch device are arranged in the same axial line so that a rotary output shaft fitted with a pinion is slidably moved in the axial direction. FIG. 3 is a cross-sectional view showing such a conventional starter motor disclosed in, for instance, Japanese Unexamined Patent Publication No. 90665/1988. In FIG. 3, "front" means the right side of each structural element shown in drawings and "rear" means the left side of the structural elements.
A reference numeral 1 designates an armature as a major element of a starter motor 100, which is mainly composed of an armature core 2 and a tubular armature rotary shaft 3 having a central bore 3a. An armature core 2 is attached to an intermediate portion of the armature rotary shaft 3 and a commutator 4 is fitted to the rear part of the rotary shaft 3. The commutator 4 is connected to an armature coil 5 which is wound in the armature core 2.
A numeral 6 designates an assembly consisting of brushes and holders arranged at the outer circumference of the commutator 4. A rear bracket 7 for a D.C. motor holds the assembly of brushes and holders 6 at the inside of the rear end portion by means of bolts. The rear bracket 7 is provided with a bearing aperture 7a extending in the axial direction at the central portion of the rear end, which is formed by bending inwardly along the axial direction. A bearing 8 is fitted to the bearing aperture 7a so as to support the rear end portion of the armature rotary shaft 3. A yoke 9 which is a part of the D.C. motor has its rear end in contact with the front end of the rear bracket 7 and attached at its inner circumferential surface with a plurality of permanent magnets 9a for producing a magnetic field to the armature. There is formed a annular shoulder portion at the outer circumference of the front end surface of the yoke, and a front bracket 10 having the corresponding annular shoulder portion at its rear end is fitted to the yoke 9. The front bracket 10 has an internal gear wheel 10a which constitutes a part of a planetary reduction gear device.
The inner diameter portion of the front bracket 10 is stepwisely reduced from the rear part to the front part to thereby form a plurality of annular shoulder portions. The front bracket 10 has also a recess for bearing 10b at the intermediate shoulder portion and the smallest diameter portion 10c formed at the front end, as well as the internal gear wheel at the rear portion of it. A sun gear wheel 11 as a flat gear wheel is formed at the outer circumference of the front end portion of the armature rotary shaft 3. Planetary gear wheels 12 are positioned between the sun gear wheel 11 and the internal gear wheel 10a to be meshed with the both gear wheels. A bearing 13 is fitted to the outer circumference of a support pin 14 which supports each of the planetary gear wheels 12.
An overrunning clutch 15 comprises a clutch inner member 15A having a helical spline teeth 15 which are formed at the inner circumferential surface at the portion near the D.C. motor and a reduced diameter portion 15b at its front portion, a clutch outer member 15B for supporting the support pins 14 at its rear part, which is disposed so as to be connectable to or detachable from the clutch inner member 15A, and rollers 15C arranged between the clutch inner member 15A and the clutch outer member 15B. A bearing 16 is fitted to the intermediate reduced diameter portion 10b of the front bracket 10 so as to bear a radial load of the clutch inner member 15A.
A rotary output shaft 17 is provided with a recess 17a at the rear end surface and spline teeth 18 at an intermediate portion, which diameter is larger than the inner diameter of the front end portion of the armature rotary shaft 3. The spline teeth 18 are fitted to the helical spline teeth 15a of the clutch inner member 15A so as to be able to move in the axial direction. A spring 19 is interposed between a shoulder portion formed in the vicinity of the front end of the spline teeth 18 and the rear end of the reduced diameter portion 15b so as to push the rotary output shaft 17 backwardly. A pinion 20 is attached to the front end portion of the rotary output shaft 17 by means of a straight spline structure 21 formed at the front portion. A stopper 22 is formed at the front end of the rotary output shaft 17 so that it retains the pinion 20 to the rotary output shaft 27 because the pinion 20 is pushed forwardly by a sping 20b which is interposed between a recess 20a formed at the rear portion of the pinion and an annular shoulder portion formed in the outer circumference of the rotary output shaft 17. The spring 20b is to move the pinion 20 backwardly along the axial line when the stopper 22 and a ring 22b are attached to the rotary output shaft 17 after the pinion 20 has been fitted to the rotary output shaft 17. Namely, the spring 20b functions to push the pinion 20 forwardly in a space which allows the movement of the pinion 20 in the backward direction after the pinion 20 has been assembled. The spring 20b also serves to reduce a shock when the pinion 20 is engaged with the ring gear of the engine. In FIG. 3, a character T denotes the thickness of the bottom portion of the pinion 20.
A sleeve bearing 23 is fitted to the inner circumferential surface of the central bore 3a of the armature rotary shaft 3 to thereby support the rear portion of the rotary output shaft 17 inserted from the front end portion of the central bore 3a so that the rotary output shaft 17 can be rotated and moved linearly in the axial direction. A bearing 24 is fitted to a bearing hole formed at the central portion of the front end of the yoke 9 so that it supports the armature rotary shaft 3 between the armature core fitting portion and the sun gear wheel 11.
An electromagnetic switch 25 is attached to the rear part of the armature 1 which is a part of a starter motor so as to provide a thrust force to the rotary output shaft 17 when the electromagnetic switch is excited. The electromagnetic switch 25 comprises a casing 26 having an opening at its front side which is firmly attached to the rear end of the rear bracket 7, a bobbin 27 with an axial opening which is positioned in alignment with the axial direction of the starter motor 100 and which is received in the casing 26, an exciting coil 28 wound around the bobbin 27 and a plunger 29 made of a ferromagnetic material which is disposed in the central aperture of the bobbin 27 so as to be movable in the axial direction, has a recess whose bottom is provided with an opening 29a which opens toward the starter motor 100, and a shoulder portion 29b at the outer circumferential surface. The electromagnetic switch 25 further comprises a core 30 having a bearing hole 30a at its central portion, which is fitted to the inner circumferential surface of the opening of the casing 26 wherein the core 30 has a shoulder portion for receiving the bobbin 27, a return spring 31 interposed between the core 30 and the shoulder portion 29a to push the plunger 29 backwardly, and a plunger rod 32 having a T-like shape in longitudinal cross section which has the front portion inserted in the central bore 3a from the rear end of the armature rotary shaft 3 wherein the plunger rod 32 is in alignment with the rotary output shaft 17 and is in contact with it through a steel ball 33 placed in the recess 17a formed at the rear end surface of the rotary output shaft 17; an intermediate portion of the plunger rod 32 is born by a bearing 34 fitted to the bearing hole 30a of the core 30, the rear end portion of the plunger rod 32 is disposed in a space which is opposite the motor 1 with respect to the core 30, and a spring 35 is arranged in the recess of the plunger 29 so as to push the plunger rod forwardly.
A movable contact (not shown) is attached to the plunger 29 through an insulating material, and a fixed contact (not shown) is attached to the casing 26 through an insulating material so as to oppose the movable contact. A lead wire (not shown) connects the movable contact to a positive terminal of a D.C. power source, while a negative terminal of the assembly of brushes and holders 6 is grounded. The positive terminal of the assembly is connected to the fixed contact by means of a lead wire. The exciting coil 28 is connected to the D.C. power source through a starter switch (not shown).
The operation of the conventional starter motor will be described. In a state that the starter switch is opened, the exciting coil 28 is not excited, whereby only the force of the spring 31 is applied to the plunger 29. Accordingly, the plunger 29, i.e. the plunger rod 32 is positioned at the rearmost position. Then, the rotary output shaft 17 does not receive the thrust force from the electromagnetic switch, so that the rotary output shaft 17 is brought to the rear position where the front end surface of the armature rotary shaft 3 comes in contact with the rear side of the spline teeth 18 by the action of the spring 19. (The position shown in FIG. 3). The front end of the plunger rod 32 of the electromagnetic switch 25 is in contact with the steel ball 33 so that the ball 33 does not come out from the recess 17a of the rotary output shaft 17. Naturally, the armature 1 is in a non-conductive state and is stopped.
By operating the starter switch, a current flows the exciting coil 28 of the electromagnetic switch 25, and the plunger 29 is moved forwardly by an electromagnetic force produced by the excitation of the exciting coil 28. Then, the movable contact is brought into contact with the fixed contact and a current flows into the brushes of the brush and holder assembly 6. The current flows from the commutator 4 to the ground via the armature coil 5. Thus, a rotating force is produced in the armature 1 by the current conduction to the armature 1. The rotating force is transmitted to the planetary gear wheels 12 through the sun gear wheel 11. The rotating force causes the revolution of the planetary gear wheels 12. A force produced by the revolution of the planetary gear wheels 12 is transmitted to the overrunning clutch 15. The revolution transmitted to the overrunning clutch 15 is transmitted to the helical spline teeth 18 through the helical spline teeth 15a of the clutch inner member 15A which is engaged with the clutch outer member 15B through the rollers 15c, whereby the rotary output shaft 17 is rotated along with the pinion 20 at a predetermined reduction ratio with respect to the rotation of the armature 1.
On the other hand, when the plunger 29 is pushed forwardly, i.e., the plunger rod 32 is pushed forwardly, the rotary output shaft 17 which receives a thrust force from the plunger rod 32 through the steel ball 33 is moved forwardly along with the pinion 20 against the spring force of the spring 19. At this moment, the spline teeth 18 move forwardly while they are engaged with the spline teeth 15a so that the relative position of engagement is changed.
The pinion 20 is projected forwardly from the reduced diameter portion 10c due to the forward movement of the rotary output shaft 17 and is engaged with the ring gear formed at the outer circumference of a flywheel which is attached to the engine. Thus, the rotating force of the armature 1 is transmitted to the ring gear at a reduced speed obtained by a pinion structure, whereby the engine is started.
As soon as the engine is started, the rotating force of the engine is transmitted to the pinion 20 through the ring gear and the rotary output shaft 17 is rotated with the pinion 20. When a speed of rotation of the rotary output shaft 17 reaches a predetermined value, the overrunning clutch 15 separates from the rotary output shaft by the action of the rollers 15c, whereby the pinion 20 and the rotary output shaft 17 rotate freely.
After the engine has been started, the starter switch is operated to be an OFF position, and then the rotary output shaft 17 is returned to the position (as shown in FIG. 3) by the returning force of the returning spring 19 without receiving any thrust force from the electromagnetic switch 25; thus, the starter is returned to the initial state (as shown in FIG. 3).
In the conventional starter motor of an overhang type, such as a coaxial type starter, a spring 20b is interposed between a shoulder portion formed in the rotary output shaft and at the radially outer portion of the straight spline structure formed in the rotary output shaft and the reduced diameter portion formed in a recess formed in the inner diameter portion of the pinion, whereby the pinion is pushed forwardly. Accordingly, in order to satisfy that the pinion should have a strength which is determined by the thickness T of the bottom portion of the pinion and the rotary output shaft should have a predetermined strength, it was necessary that the number of teeth of the pinion should be a predetermined number or greater. Namely, it was necessary that the diameter of the pinion is a predetermined value or greater. In D.sub.p 10 (M=2.54 in module) which has been generally used for ring gears and pinions for automobiles, the minimum value is 8.
On the other hand, the volume of the armature of a motor is in inverse proportion to the gear ratio of a pinion to a ring gear. There has been a demand of reducing the volume of the armature, i.e., miniaturizing or reducing the weight of the motor by increasing the gear ratio between the pinion and the ring gear while the number of teeth of the ring gear should have a predetermined value and the number of teeth of the pinion should be reduced. However, it was impossible to reduce the number of teeth of the pinion because of the reason described above.
In the conventional coaxial type starter, however, it was difficult to reduce the length in the axial direction of the starter because the electromagnetic switch is attached to the rear of the motor, owing to the same reason described above.
In the conventional starter motor, the helical spline structure constitutes a supporting point to a load applied to the rotary output shaft. Accordingly, the span between the pinion and the supporting point was inevitably long and the rotary output shaft was apt to incline, whereby there were problems of occurrence of noises and the weakening of the strength of the shaft.